One night stand with about a 260lbs, she sat on my face for a long time. Simply asked and without question or concern she climbed on. First on a waterbed, i remember hitting the bottom of my head...hehe. Was so good we did it a few more time, on the couch, the floor.
I weigh 147lbs, lean. Because my weight is mostly muscle, it's a lot more compact/dense and thus feels much heavier. Steed is a strong guy (competitive bodybuilder, in fact), but even he gets tuckered out when I ride him ('pony-play') or sit on him.
The only time I don't do full-weight face-sitting is when I want him to clean me or bring me to an orgasm - I'll usually hover a bit for that so he has better access. Otherwise, it's my full weight on his face.
I admit it's quite amusing - his face gets beet red after a nice, long session and if I'm wearing panties, jeans, yoga/weightlifing pants, etc he'll have a perfect imprint of them on his face for a good ten minutes. I do believe the term 'panty-face' is appropriate here. I love calling him that, ha!